A. Field of Invention
The present invention generally relates to high speed electric motors and more particularly to technology that decreases torque ripple in high speed reluctance motors.
B. Description of the Related Art
High speed motors are highly desirable for a variety of applications. For example, high speed motors can eliminate the need for a transmission mechanism to attain the necessary high operating speeds for machinery or vehicles. As a result, the machine or vehicle can operate more efficiently because power loss due to friction within the transmission mechanism is eliminated. In particular, such high speed motors are highly desirable for spinning machines used in the looming industry in which high operating speeds are necessary in order to achieve high product throughput.
High speed reluctance motors are well known and particularly suited for such applications. These electro-magnetically driven motors are characterized by having a set of electromagnets (usually conducting coils wrapped around iron projections within the stator of the motor) that are energized and de-energized in a manner that rotates an iron rotor positioned in the center of the motor. Such known reluctance motors are generally driven with rectangular shaped waveform pulses.
In such reluctance motors, no torque ripple will occur in theory, if the stator coils are driven with precise rectangular waveforms. However, because electric current can not be changed instantaneously so as to generate perfectly rectangular waveforms, torque ripple is generated in known reluctance motors when the motor is rotated at a high speed.
Torque ripple results in vibrations and as torque ripple increases, known reluctance motors are subject to malfunction, the rotor can not rotate smoothly in unison with the phase changes of the stator coils. Therefore, the motor can not operate at the intended speed. In addition, the operating noise of the motor resulting from uneven revolution speed may be increased.
Various types of technologies have been proposed in order to decrease torque ripple in high speed reluctance motors. For example, in Japanese Laid-Open Patent Publication No. 8-322171, torque ripple is purportedly decreased by incorporating a skew structure on the rotor poles. In Japanese Laid-Open Patent Publication No. 8-126273, torque ripple is purportedly reduced by forming slits within each rotor pole and a pair of projections are formed at both sides of the tip of each rotor pole.
However, in the first reference, the process for forming the rotor requires a number of steel plates having difference cross-sections to be laminated and such manufacturing process is complicated. Additionally, in both systems, torque ripple is not actually suppressed as the revolution speed of the motor is further increased. Thus, a long-felt need exists to design a high speed reluctance motor in which torque ripple is substantially or completely suppressed at high operating speeds and is economical to produce. Such a reluctance motor would provide an inexpensive and reliable source of high speed power.
It is therefore an object of the present invention to provide an improved reluctance motor that overcomes, or at least substantially minimizes, the problems of the prior art.
Such problems of the prior art can be overcome by designing a reluctance motor in accordance with a defined relationship between the various parts of the motor. In its broadest aspect, the width or spread angle of the rotor poles is designed to be greater than the width or spread angle of the stator poles. Preferably, the relationship of the widths or spread angles of the rotor poles and the stator poles is within a preferred range. Techniques for calculating this preferred range are taught herein.
In the alternative, a reluctance motor is provided in which the stator coils are driven with sine waveforms, or waveforms substantially similar to sine waveforms. Because sine waveforms are relatively easy to generate in comparison to perfect rectangular waveforms, a reluctance motor with little or no torque ripple, and therefore little or no vibration, at high operating speeds can be achieved.
Optionally, both of these independent features may be combined to construct reluctance motors. Such motors have a defined relationship between the widths or spread angles of the stator poles and the rotor poles and are driven with sine, or substantially sinelike, waveforms. Such reluctance motors can be constructed using known techniques for manufacturing reluctance motors to achieve a low-cost, quiet and highly reliable high speed motor.
Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims.